Every year hundreds of millions of rubber truck and car tires are worn out and discarded. In the past, and continuing to a lesser degree today, the tires were buried in underground dumps. However, rubber does not degrade and will therefore remain in the ground forever and cause an environmentally undesirable situation. Moreover, given the buoyancy of the tires they tend to rise to the top of the ground regardless of the location in which they are buried and ultimately cause an unsightly and disruptive protrusion through the surface of the ground.
Systems have therefore been developed to attempt to recycle the tires into a useable form for incorporation into new tires, or for reduction into pellets or crumb rubber which can be used for a variety of purposes, including use as a fuel source. A problem encountered by burning tires as a fuel source is that the tires in their original form are extremely bulky and costly to store and transport, and in addition, make for relatively inefficient combustion given their shape. It is therefore advantageous to reduce the tires into small particles or crumb rubber on the order of a few millimeters in cross section.
Machines are therefore in existence which shred, or tear, or in some other way reduce the tires into smaller pieces for eventual use as a fuel source. One such example is disclosed in U.S. Pat. No. 5,411,216. As disclosed therein, a pair of rotating shafts are provided with teeth which intermesh as the shafts rotate. The rubber is torn apart as the shafts rotate against each other in opposite directions, literally pulling the rubber apart. However, by employing a pulling action a relatively large amount of power is required to pull the rubber in opposite directions until the pulling force is beyond the tensile strength of the rubber, and the rubber effectively "snaps" apart. Such a process results in non-uniformly sized particles.
Another example of such a system is disclosed in U.S. Pat. No. 5,511,729, issued to Husain, which also reduces the rubber in the form of rotating shafts which have intermeshing teeth designed to pull the rubber apart. The Husain reference provides pockets in each tooth to prevent clogging of the system, but again the horsepower requirements of the system are substantial.
To put the horsepower requirements in more quantitative terms, a typical system which "pulls" the rubber apart will normally require a minimum of 5,000 horsepower to produce approximately 6,000 lbs of fine crumb rubber per hour from tire stock. Given the ever increasing energy costs of most locations, the cost for powering 5,000 horsepower places a huge burden on the eventual cost effectiveness of tire recycling.
Moreover, by recycling tires in a manner similar to the devices mentioned above, an enormous physical burden is placed on the actual teeth of the rotating shafts which causes the teeth to become dulled and eventually unusable in relatively short order. This situation is made even more problematic by the fact that the process of resharpening or replacing the toothed shafts is an extremely time consuming and costly measure. For example, it can take a minimum of 8 hours to replace or regrind a cutting shaft, and in most situations this operation needs to be performed at least monthly. This results in an annual maintenance cost on the order of hundreds of thousands of dollars.